RS485 - difficult to place line terminal resistor - alternative solutions viable ?

[Grossel]

In a facility , there are about 5m (15 ft) between a modbus master and slave1 (there is only those two devices to be connected to each other this time).

Since this facility is kind of - takes time to go there - I want to make the best possible RS485 bus.

There are expected to be some low frequency noise in the nearby area, and the baud rate will be set to 9600bps (but I want it to support higher in case of more equipment get added to the rs485 bus later on).

The issue is that the modbus master are manufactured with one RJ45 socket for RS485 instead of simple screw terminal. This makes it difficult to make a proper cable (adding a resistor very close to the RJ45 plug just create a weak spot and somewhere a jacket or strap may get caught). This is not wanted as a solution.

The alternatives:

1. Assumed best for signal. But will cause a clump and weak spot on the wire that is not wanted.
2. Tap wire for modbus master. Assuming almost as good. However I cannot find (or phrase the right question for AI to understand) any source online that specify the lowest ratio between length of tap wire (closest to end) and the last wire segment to where the end resistor are connected. I.e. Can the ratio be as low as 1:1 or should the wire segment closest to the end be longer than some given distance ?
3. Insert a separate screw terminal bus bar for connecting the resistor. This is the method I would want to use (because least amount ow work). But then then there will be a wire segment after the terminal resistor, about 40cm (1 1/3 ft) in order to reach around the master device.

Question: What do you guys think about alternative 2 - making a tap wire in order to place the terminal resistor on the true end of the rs485 bus ?

 

[ahsrabrifat]

The bus bar method is the most robust solution and should work well with no weak spots. The 40cm wire after the resistor shouldn't be a major issue at 9600bps, but you could test the system to confirm. If the setup grows, this method offers the flexibility to add additional devices without much hassle. You can go with the separate screw terminal bus bar solution for long-term reliability, especially if you're looking to expand the RS-485 network later.

 

[rjenkinsgb]

A 40cm stub is fine. The recommended maximum stub length for 1MBit CAN is 30cm, so you you dont't need to worry about any standard serial port speed .

 

[Grossel]

Thanks for answer.

I could test the system at 9600bps - but I also should take in account for more noise from nearby environmental sources if/when the facility expand in the future.

ahsrabrifat - I assume you by "bus bar method" means alternative 3 in the drawing?